Organic light-emitting diode (OLED) technology incorporates organic luminescent materials that, when sandwiched between electrodes and subjected to a DC electric current, produce intense light of a variety of colors. These OLED structures can be combined into the picture elements or pixels that comprise a display. OLEDs are also useful in a variety of applications as discrete light-emitting devices or as the active element of light-emitting arrays or displays, such as flat-panel displays in watches, telephones, laptop computers, pagers, cellular phones, calculators, and the like. To date, the use of light-emitting arrays or displays has been largely limited to small-screen applications such as those mentioned above.
Demands for large-screen display applications possessing higher quality and higher resolution has led the industry to turn to alternative display technologies that replace older LED and liquid crystal displays (LCDs). For example, LCDs fail to provide the bright, high light output, larger viewing angles, and high resolution and speed requirements that the large-screen display market demands. By contrast, OLED technology promises bright, vivid colors in high resolution and at wider viewing angles. However, the use of OLED technology in large-screen display applications, such as outdoor or indoor stadium displays, large marketing advertisement displays, and mass-public informational displays, is still in the development stage.
Several technical challenges exist relating to the use of OLED technology in a large-screen application. One such challenge is that OLED displays are expected to offer a wide dynamic range of colors, contrast, and light intensity depending on various external environmental factors including ambient light, humidity, and temperature. For example, outdoor displays are required to produce more white color contrast during the day and more black color contrast at night. Additionally, light output must be greater in bright sunlight and lower during darker, inclement weather conditions. The intensity of the light emission produced by an OLED device is directly proportional to the amount of current driving the device. Therefore, the more light output needed, the more current is fed to the pixel. Accordingly, less light emission is achieved by limiting the current to the OLED device.
A pixel, by definition, is a single point or unit of programmable color in a graphic image. However, a pixel may include an arrangement of sub-pixels, for example, red, green, and blue sub-pixels. There are two basic circuit configurations for driving these sub-pixels, namely, a common cathode configuration and a common anode configuration. These configurations differ as to whether the three sub-pixels are addressed via a common cathode line or addressed via a common anode line, respectively. Accordingly, in the common cathode configuration, the cathodes of the three sub-pixels are electrically connected and addressed in common; in the common anode configuration, the anodes of the three sub-pixels are electrically connected and addressed in common.
Conventional OLED displays typically use the common cathode configuration. In a typical common cathode drive circuit, a current source is arranged between each individual anode and a positive power supply, while the cathodes are electrically connected in common to ground. Consequently, the current and voltage are not independent of one another, and small voltage variations result in fairly large current variations, having the further consequence of light output variations. Furthermore, in the common cathode configuration, the constant current source is referenced to the positive power supply, so again any small voltage variation will result in a current variation. For these reasons, the common cathode configuration makes precise control of light emission, which is dependent upon precise current control, more difficult.
By contrast, in a typical common anode drive circuit, a current source is arranged between each individual cathode and ground, while the anodes are electrically connected in common to the positive power supply. As a result, the current and voltage are completely independent of one another, and small voltage variations do not result in current variations, thereby eliminating the further consequence of light output variations. Furthermore, in the common anode configuration, the constant current source is referenced to ground, which does not vary, thereby eliminating any current variations due to its reference. For these reasons, the common anode configuration lends itself to the precise control of light emission needed in a large-screen display application.
Another consideration in a large-screen display application using OLED technology is the physical size of the pixel. A larger emission area is more visible and lends itself to achieving the required wide dynamic range of colors, contrast, and light intensity. Consequently, an OLED device structure having a larger area than OLED structures of conventional small-screen displays is desirable. In a small-screen application, the pixel pitch is typically 0.3 mm or less and the pixel area is, for example, only 0.1 mm2. By contrast, in a large-screen application, the pixel pitch may be 1.0 mm or greater, thereby allowing the pixel area to be as large as 0.3 to 50 mm2 (pitch varies up to 10 mm or more with fill factors of 50%). However, a consequence of the larger device area is the relatively high inherent capacitance (COLED) of the larger OLED device as compared with small OLED structures. Due to this high inherent capacitance, in operation, an additional amount of charge time is required to reach the OLED device working voltage. This charge time limits the on/off rate of the device and thus adversely affects the overall display brightness and performance.
OLED pre-charge circuits have been developed and integrated into the existing drive circuitry to help overcome the capacitance characteristic of OLEDs within a graphics display device. For example, U.S. Pat. No. 6,323,631, entitled, “Constant current driver with auto-clamped pre-charge function,” describes a constant current driver with auto-clamped pre-charge function that includes a reference bias generator and a plurality of constant current driver cells, each being connected to the reference bias generator to form a respective current mirror. Each constant current driver cell has a switch transistor, a current output transistor, and a pre-charge transistor. When a constant current is output from the current output transistor for driving an OLED, the pre-charge transistor is turned on to provide a drain to source current as an additional large current for rapidly pre-charging the OLED until the gate to source voltage of the pre-charge transistor is smaller than the threshold voltage. While the pre-charge function of the '631 patent suitably serves to rapidly pre-charge the OLED devices and thereby optimize performance, the pre-charge function of the '631 patent is designed for use in a common cathode drive circuit and is therefore not suitable for use in the common anode drive circuit of a large-screen OLED display device. A further drawback of the pre-charge function of the '631 patent is that it is designed to handle the COLED value associated with a small pixel area, such as 0.1 mm2, and is therefore not able to overcome the larger COLED value associated with a large pixel area.
It is therefore an object of the invention to provide a pre-charge circuit suitable for use in a large-screen OLED display arranged in a common anode configuration.
It is another object of this invention to provide a pre-charge circuit suitable to overcome the large COLED value associated with the large-area OLED device of a large-screen OLED display arranged in a common anode configuration, thereby optimizing performance.
It is yet another object of this invention to provide a pre-charge circuit that eliminates the effects of varying OLED device characteristics, such as capacitance, due to manufacturing process variations.